Thermoplastic films have been used for many years in connection with packaging of various articles including food products such as fresh red meat. The latter is often in the form of large pieces (primal cuts) which are placed in bags formed of thermoplastic films which are evacuated and sealed as for example by heating the opposite surfaces of the bag at the open mouth end and pressing the inner surfaces together to form a heat seal.
The resulting food-containing packages are often shipped from the packing house to supermarkets or retail butchers where the packages are opened and the primal cuts divided into smaller pieces for retail packaging. These food-containing packages must provide an oxygen-free environment for substantial periods such as 4-8 weeks, so the film packaging art has provided multilayer films with a core layer which is an oxygen barrier material. The most commonly used oxygen barrier materials are vinylidene chloride copolymers with various comonomers such as vinyl chloride (VD-VDC copolymer) or methyl acrylate (MA-VDC copolymer). Other known oxygen barrier materials include polyamides and ethylene vinyl alcohol copolymer (EVOH).
In addition to the oxygen barrier function, the thermoplastic film has several other requirements as for example resistance to abuse and stress during packaging and handling, an attractive uniform appearance with no streaks, and good optical properties so that the valuable food product such as a fresh red meat primal cut may be periodically visually inspected to insure that the package integrity has not been lost. Accordingly, good optical properties include low haze and high gloss.
From a functional standpoint, the thermoplastic film must provide high abuse resistance because the food-containing packages are often moved several times in the shipping-distribution chain and subjected to external abuse and impact. Also, the inside surface usually must be heat sealable to itself, and since different heat sealing apparatus often operate at different temperatures, the inside surface must be heat sealable over a wide temperature range.
Another physical requirement of suitable thermoplastic films for packaging oxygen sensitive products such as fresh red meat, is that the film must be heat shrinkable in both the machine direction (MD) and the transverse direction (TD). This is necessary so that the food-containing film package may be evacuated with the film inner wall collapsing against the food product outer surface, and thereafter heated as for example by hot water spray in a tunnel to heat shrink the collapsed film and provide a tight package.
Because of these numerous requirements, thermoplastic films commonly have at least three layers; the aforementioned core-barrier layer, an abuse resistant layer on one side of the core layer and a heat sealing layer on the other side of the core layer.
Heretofore, a commonly used multilayer film for packaging fresh red meat has been a three layer type comprising a vinylidene chloride copolymer barrier layer and polyolefin layers on opposite sides, most commonly polyethylene-ethylene vinyl acetate (EVA) blend abuse and heat sealing layers. The preferred polyethylenes are very low density polyethylene (VLDPE) which is also called ultra low density polyethylene (ULDPE), and linear low density polyethylene (LLDPE).
In part because of increasing emphasis by governmental authorities on incineration disposal of waste materials without generation of chlorine-containing gases, there is a need to identify a nonchloride type oxygen barrier film.
Another reason why there is a need to replace vinylidene chloride copolymer type oxygen barrier layers is their well known tendency to partially degrade and discolor when exposed to substantial doses of irradiation. Irradiative crosslinking is commonly said to improve the puncture resistance of the polyethylene in the inner and outer layers, or improve the tensile strength of the EVA in the inner layer to permit biaxial orientation by forming a stable bubble of the primary tube, or to widen the heat sealing range of the inner layer, or a combination of the above. When VC-VDC copolymer is irradiated at doses in the order of 5 MR, the material partially degrades and develops a yellowish tint which is aesthetically unpleasing.
EVOH has been known for many years as a suitable oxygen barrier material, and in fact is commercially used in certain food packaging applications as for example, cook-in systems wherein the fresh food body is placed in a bag which is evacuated and sealed. The package is then placed in a hot water bath and the food body cooked insitu. After cooking the package is cooled and stored at low temperature until ready for use. In general, these cook-in packages need not be heat shrinkable to the extent required in the fresh red meat market, and are not subjected to the physical abuse experienced by fresh red meat packages.
To the best of our knowledge, EVOH-based oxygen barrier layer films have not been widely used in commerce for the packaging, sorting and shipping of fresh red meat. One reason is that EVOH is far more sensitive to moisture contact than vinylidene chloride copolymers. That is, the oxygen barrier quality of EVOH rapidly and irreversibly declines when exposed to significant quantities of water. This means that the EVOH layer must be extremely well protected against moisture intrusion from the stored product through the inner layer(s) and also from the atmosphere through the outer layer(s).
Another reason why EVOH has not been widely used as an oxygen barrier layer in thermoplastic bags for packaging fresh red meat is that its adhesion properties to polyethylene-EVA blends are substantially inferior to vinylidene chloride copolymers. Whereas the latter may be adhered directly to polyethylene-EVA blends with sufficient strength to withstand delamination forces during heat shrink, this has not been possible with EVOH-based oxygen barrier layers. Instead, it has been necessary to add additional layers between the barrier layer and the abuse or heat sealing layers. These extra layers are known as adhesive to tie layers and perform a function of interlayer adhesion between the EVOH layer and the (outer) abuse resistant layer or the (inner) heat sealing layer in the bag formed from the multilayer thermoplastic film. The materials forming these adhesive layers are chemically complex, as for example polyethylene-based modified maleic anhydride types, hence expensive. Moreover they add to the complexity and expense of the manufacturing process.
A further reason why EVOH multilayer films have not replaced vinylidene chloride copolymer films is that the former are far more sensitive to biaxial orientation conditions, and the range of satisfactory processing conditions is much narrower.
Still another reason why EVOH-based oxygen barrier layers have not replaced vinylidene chloride copolymer types is that EVOH resin cost is much higher. Typically the EVOH-based layer in presently known multilayer films comprises between about 8 and 20% of the total film thickness, and is the most expensive material in the film.
An object of this invention is to provide a biaxially stretched heat shrinkable multilayer film with an EVOH-based oxygen barrier layer, and having physical properties at least equivalent to vinylidene chloride copolymer barrier layer films currently used to package fresh red meat.
Another object is to provide such an EVOH-based barrier layer type film with a barrier layer substantially thinner than known EVOH type multilayer films proposed for fresh red meat packaging.
A further object is to provide such an EVOH-based barrier layer type film having fewer than six layers.
A still further object is to provide such an EVOH-based barrier layer type film with at least equivalent optical properties to vinylidene chloride copolymer barrier type multilayer films currently used for fresh red meat packaging.
Other objects and advantages of the inventive film will be apparent from the ensuing disclosure and appended claims.
U.S. Pat. No. 4,407,897 to Farrell et al describes a multilayer film comprising an EVOH core layer, intermediate modified polyolefin adhesive layers, and polyolefin outer layers.
U.S. Pat. No. 4,495,249 to Ohya et al describes a biaxially stretched heat shrinkable multilayer film comprising an EVOH core layer, intermediate adhesive layers formed of carboxylic acid modified polyethylene, and EVA-LLDPE blend outer layers.
U.S. Pat. No. 4,557,780 to Newsome et al describes a biaxially stretched heat shrinkable multilayer film comprising a 0-50% polyamide e.g. nylon 6,66 and 50-100% EVOH core layer, intermediate adhesive layers formed of olefinic polymers having carboxyl modifications, and outer layers comprising 40-100% EVA and 0-60% LLDPE.
U.S. Pat. No. 4,615,926 to Hsu et al describes a multilayer film comprising an EVOH-polyamide core layer, olefin based adhesive intermediate layers, an ionomer inner layer and a polyethylene outer layer.
U.S. Pat. No. 4,758,463 to Vicik describes a three layer biaxially stretched heat shrinkable film suitable for meat cook-in applications, having an EVOH-polyamide core layer and outer layers comprising a blend of EVA and an EVA anhydride functionally adhesive compound. This type film is not suitable for use in fresh red meat packaging because of its relatively low puncture resistance and relatively high cost EVOH core layer.
U.S. Pat. No. 4,851,290 to Vicik discloses a three layer irradiated film for holding hot-filled foods during the succeeding cold water tumble-chilling and further refrigerating steps, comprising a nylon 6,12 or nylon 6,66 core layer, and outer layers comprising a blend of 50-75% polyethylene or EVA and 25-50% polyethylene-modified adhesive or EVA modified adhesive.
U.S. Pat. No. 4,857,399 to Vicik describes a four layer biaxially stretched heat shrinkable film suitable for nonadhering meat cook-in applications, having an EVOH-polyamide core barrier layer, a blend of EVA and anhydride-modified ethylene copolymer adhesive as an outer abuse layer on one side of the barrier layer, an EVA-anhydride-modified ethylene copolymer blend as a layer on the other side of the barrier core layer, and an inner or meat contacting layer comprising an ethylene-propylene random copolymer. This type film is not suitable for use in fresh red meat packaging because of its relatively poor optical properties and relatively low shrink.
U.S. Pat. No. 5,004,647 to Shah describes a biaxially stretched heat shrinkable film having an 80-99% EVOH and 1-20% polyamide blend core layer, intermediate adhesive layers for example formed of anhydride-modified polyethylene, and three component blend outer layers of LLDPE, linear medium density polyethylene (LMDPE) and EVA.
U.S. Pat. No. 5,075,143 to Bekele describes a nine layer film having an EVOH core layer, intermediate EVA layers, adhesive layers formed of chemically modified polyolefin, and moisture resistant layers formed of ionomer or VLDPE. The outer layers are a VLDPE sealant layer on the inside, and a heat resistant HDPE layer on the outside.